1. Field of the Invention
The invention relates to a capacitive fingerprint sensor and a method for manufacturing the same, and more particularly to a capacitive fingerprint sensor against ESD damage and contamination interference and a method for manufacturing the same.
2. Description of the Related Art
There are many known techniques of identifying an individual through the identification of the individual's fingerprint. The use of an ink pad and the direct transfer of ink by the thumb or finger from the ink pad to a recording card is the standard way of making this identification. Then, an optical scanner scans the recording card to get an image, which is then compared to fingerprint images in the computer database. However, the most serious drawback of the above-mentioned method is that the fingerprint identification cannot be processed in real-time, and thus cannot satisfy the requirement of real-time authentication, such as network authentication, e-business, portable electronics products, personal ID card, security system, and the like.
The method for reading a fingerprint in real-time has become the important technology in the biometrics market. Conventionally, an optical fingerprint sensor may be used to read a fingerprint in real-time. However, the optical fingerprint sensor has a drawback because it is large in size.
Consequently, silicon fingerprint sensors, which overcome the drawbacks of the optical sensor and are formed by silicon semiconductor technology, are developed. According to the consideration of silicon integrated circuit (IC) processes, the capacitive fingerprint sensor has become the most direct and simple product.
Structurally speaking, the capacitive fingerprint sensor includes a plurality of capacitive sensing members arranged in a 2D array. When a finger contacts the exposed surfaces of the sensing members, capacitance curves corresponding to the ridges of the finger may be sensed. Because most of the sensor surface has to be exposed to the outside, any approaching charged body may cause shorted circuit or permanent damage to the circuit in the sensor. Consequently, it is quite important for the sensor surface to possess the ability against the electrostatic discharge (ESD) damage. In addition, it is necessary to prevent the contamination interference from influencing the captured image quality.
A method for avoiding the ESD damage to the fingerprint sensor is disclosed in U.S. Pat. No. 6,114,862 to Tartagni et. al. and U.S. Pat. No. 6,515,488 to Thomas, the disclosures of which are hereby incorporated by reference.
Referring to FIGS. 1 to 3, the above-mentioned method for avoiding the ESD damage utilizes a tungsten metal mesh 113, which surrounds each capacitive sensing member and is exposed to the outside, to conduct the electrostatic charges of an approaching body to the ground, which method may effectively solve the problem of ESD damage. However, the method and design for forming the tungsten metal mesh 113 may cause other problems. The plate electrodes 112 and the tungsten metal mesh 113 are positioned above the substrate 110 at different levels and are formed by different materials in different manufacturing processes. After the steps of depositing and then etching back the tungsten metal, several small cavities 108, which are regarded as defects and may cause problems such as stress concentration and the like, are formed on the protection layer 111 of the sensor surface. When the fingernail unintentionally hits the external surface 109 of the sensor, the sensor may be damaged. Furthermore, the small cavities 108 make the surface of the protection layer hydrophilic. Thus, the moisture of the finger tends to diffuse after the finger contacts the external surface 109, and the image quality is deteriorated accordingly. Consequently, Thomas discloses a method for filling the small cavities 108 with the silicon oxide 107 so as to make the external surface 109 smooth by depositing the silicon oxide 107 followed by the CMP process. However, this way makes the manufacturing processes too complicated and is not suitable for the general manufacturing procedures of the commercial IC foundry.
Furthermore, the above-mentioned technology also causes the problem that the finger contamination may interfere the image quality. As shown in FIG. 2, when the finger 1, which may be regarded as a virtual ground, contacts the external surface 109 of the sensor, the sensed capacitance formed between the ridge 11 and the plate electrode 112 is greater than that formed between the valley 12 and the plate electrode 112. The electrostatic charges of the finger 1 may be discharged via the tungsten metal mesh 113, and the fully exposed tungsten metal mesh 113 connects the residual contamination 114 of the finger to the ground. Then, a contamination capacitance 115 is indirectly formed to interfere the image quality, as shown in FIG. 3.